High capacity rechargeable batteries currently on the market utilize lithium-ion and iron cells. Because of the high energy densities associated with these cells, there are safety and stability issues. If overcharged, a lithium-ion battery with excess charging capacity may overheat resulting in possible rupture or explosion. If discharging becomes excessive, the battery's durability and ability to maintain a charge may degrade and deteriorate over time. Likewise, iron batteries exhibiting excessive discharges may also lead to early degradation and reduced battery life. Thus, there is a need for a protection circuit for rechargeable batteries to prevent overcharging or excessive discharging in order to minimize battery degradation and shortened battery life.
When dual source rechargeable battery operates in series, a balancing circuit can be employed to minimize the charging capacity differences between the two ends. One of the present balancing methods utilizes energy dissipation, whereby the end of the battery with the higher charged capacity discharges through a discharge logic loop to reduce its charging capacity and achieve equilibrium with the end having lower charged capacity. However, the time it takes for the two ends to reach equilibrium becomes restricted by the discharge logic loop current resulting in lowered discharge current. And as the time it takes to balance the two ends increases, the effectiveness of the balancing circuit decreases resulting in loss of energy.
FIG. 1 illustrates an existing balancing circuit and its working principles. As shown, the balancing circuit starts with a balanced reference value Vref. When a portion of the voltage is higher or lower than Vref, the balancing circuit is activated. The portion of the voltage that exceeds Vref is transmitted to a discharge circuit or logic loop to release the excess voltage. When both sections of the battery have voltages that are higher than or lower than Vref, the balancing circuit is deactivated and nothing passes through the discharge circuit and no excess voltage is released.
Tracing the specific path of the circuit, comparing both sections of the battery after passing through a comparator with the balance reference value Vref, four output states of the comparator correspond to four actual situations, the signals controlling the P-channel metal oxide semiconductor (PMOS) passing through the logic loop can be calculated by the NAND logic operator while signals controlling the N-channel metal oxide semiconductor (NMOS) passing through the logic loop can be calculated by the AND logic operator:
I. Both sections of the battery cell being higher than Vref and both comparator outputs being 1. From the logic operations in the figure above, N_out is 0 and P_out is 1; the balancing circuit is not activated.
II. Battery 1 being higher than Vref, battery 2 being lower than Vref, C1 output is 0 and C2 output is 1. Based on the logic operations, N_out is 1 and P_out is 1; battery 1's balancing bypass discharge circuit is activated.
III. Battery 2 being higher than Vref, battery 1 being lower than Vref, C1 output is 1 and C2 output is 0. Based on the logic operations, N_out is 0 and P_out is 0; battery 2's balancing bypass discharge circuit is activated.
IV. Both sections of the battery cell being lower than Vref and both comparator outputs being 0. From the logic operations in the figure above, N_out is 0 and P_out is 1; the balancing circuit is not activated.
Although this circuit can achieve a certain degree of voltage balancing, its effectiveness depends on the magnitude of the current within the resistor of the discharging bypass circuit. As such, the bypass circuit has poor efficiency and suffers loss of energy due to the resistance. Furthermore, excessive use may cause an increase in resistor temperature and overheating of the battery leading to safety issues.